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A First: Directing Heat in Solids

A device that controls the direction of heat flow could one day have valuable uses in microelectronics and energy-efficient buildings.

Scientists have been precisely controlling electric current for decades, building diodes and transistors that shuttle electrons around and make computers and cell phones work. But similarly controlling the flow of heat in solids stayed in the realm of theoretical physics–until now.

Alex Zettl and his colleagues at the University of California, Berkeley (UC Berkeley), have shown that it is possible to make a thermal rectifier, a device that directs the flow of heat, with nanotubes. If made practical, the rectifier, which the researchers described in last week’s Science, could be used to manage the overheating of microelectronic devices and to help create energy-efficient buildings, and it could even lead to new ways of computing with heat.

Heat can be thought of as made of tiny packets of vibrations called phonons. A thermal rectifier allows phonons to move only in one direction. But rectifying heat is much harder than rectifying electric current, because most heat transport in materials is by diffusion, which is random, says Arunava Majumdar, a mechanical engineer at UC Berkeley and a coauthor of the Science paper. “The vibrations are not charged, so you can’t bias them.”

The nanotube thermal rectifier is the first experimental proof that such a device works. “From that point of view, I think it’s really interesting,” says Michel Peyrard, theoretical physicist at the Ecole Normale Superieure, in Lyon, France. Four years ago, Peyrard and a team of fellow European physicists first proposed a plan for making a thermal rectifier. The basis of the European physicists’ scheme was that dissimilar materials conduct heat in various ways at different temperatures. They proposed making a thermal rectifier by combining such materials. But they never ventured into experimental physics to demonstrate their idea.

Other theorists have suggested that if a one-dimensional system has more mass at one end, heat would flow from that end to the low-mass end. Zettl’s graduate student Chih-Wei Chang decided to test this theory by making rectifiers with nanotubes, because their extremely narrow width makes them nearly one-dimensional. Plus, they’re known to be efficient heat conductors.

Chang made carbon nanotubes and boron-nitride nanotubes between 10 and 40 nanometers wide. He placed individual nanotubes in a test chamber, with each end of the tube bonded to silicon and platinum-based electrodes that act as either heaters or sensors. Then he deposited a platinum compound unevenly along the length of the nanotube so that the tube had more mass at one end than at the other. Finally, he sent a known amount of power through the heater end and measured the change in temperature at both electrodes to see how much heat was passing through the nanotube. As it turned out, as many as 7 percent more phonons were traveling from the high-mass end to the low-mass end than in the opposite direction.

This small efficiency is not enough for practical applications. But what’s more important at this stage is the solid proof that the thermal-rectifying effect exists, says Giulio Casati, a professor of physics at the University of Insubria, at Como, Italy, who, along with Peyrar, originally proposed the idea of a thermal rectifier. “It’s the first step,” he says. “When [engineers] built the first electrical diode, the efficiency was very low. So it’ll take time.”

Majumdar says the next step is to explore various nanotube geometries as well as the platinum compound loaded on the nanotubes. “Could we change that material or could we change the geometry and thereby increase rectification?” he asks. “That’s still up for grabs right now.”

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